There is a long term trend of increasing interest in active and passive seismic monitoring in and around oil and gas fields. For a summary, see, for example, Weijers, L. Advanced Fracture Methods and Mapping, Soc. Petroleum Engineers training course (2005). The recording of seismic data on the surface of the earth, in arrays of shallow wells, and in deep boreholes has been utilized. The discrimination of compressional waves from shear waves is an integral part of applications to determine rock and fluid properties. In the monitoring of hydrofracturing of producing oil and gas wells, it can be useful to be able to discriminate between compressional and shear waves.
Techniques such as described in U.S. Pat. No. 5,774,419 are used to detect seismic arrival events from background noise. Techniques such as described in U.S. Pat. No. 7,663,970 are utilized to locate seismic source events. Techniques such as described in U.S. Pat. No. 7,660,194 B2 are used to refine the seismic velocity field to enhance the location of seismic source events. Techniques such as described in U.S. Pat. No. 7,590,491 B2 are used to passively monitor production of fluids from reservoirs.
Techniques for 3D and 4D seismic surveys of oil and gas fields using arrays of sensors and active seismic sources deployed on the surface are well established in commercial practice. Also, in recent practice, permanent deployments of 3C linear sensors and pressure sensors in arrays of shallow monitoring wells have become a common commercial practice over selected oil and gas fields. These deployments are used for active monitoring utilizing active seismic sources; and for passive monitoring to detect natural seismic events that may in turn be due to movement of fluids, hydrofracturing, or the like.
Techniques have been devised to attempt to separate compressional and shear waves in the processing of multi-component linear motion data. These include many various well established seismic signal and image processing techniques, as well as wave propagation based processing, such as, for example that described in Sun, R. et. al., Separating P- and S-waves in prestack 3D elastic seismograms using divergence and curl, Geophysics, vol. 69, no. 1, pp. 286-297 (2004).
It is well understood in many fields of physical science and engineering that a complete representation of mechanical motion requires the measurement of six degrees-of-freedom. Typically this is accomplished by measuring three orthogonal linear motions, and measuring rotations around three orthogonal axes.
There is a well established technology for measurement of the linear particle motion of seismic wavefields in the earth. Many commercial sensors exist to measure particle velocity or particle acceleration along one, or up to three, linear axes, utilizing various physical concepts to accomplish the measurements. It is most common to utilize measurements of the vertical particle motion.
There is an evolving commercial technology for measurement of the rotational particle motion of seismic wavefields in the earth. Early technology is represented by, for example, U.S. Pat. No. 3,407,305 and U.S. Pat. No. 4,603,407. Newer technology is represented by, for example, sensors such as those commercially offered by MetTech (model Metr-3), June, 2010, http://www.mettechnology.com/ and Eentec (models R-1 and R-2), June 2010, http://www.eentec.com/R-s_data_new.htm. U.S. Pat. No. 7,516,660 B2 describes MetTech sensor technology. U.S. Pat. No. 7,474,591 B2 describes technology to measure rotational data from differences of linear data.
Seismic rotational motion is commonly understood to be the vector curl of the infinitesimal displacement field. The existing rotational sensors are understood to measure the components of this vector curl.
The utility of rotational seismic measurements is appreciated in earthquake and regional crustal seismology, as discussed, for example, in Lee, W., et. al., Rotational Seismology and Engineering Applications, Bull. Seismological Society of America, vol. 99, no. 2B, supplement (May 2009).
The free surface of the earth adds a significant complicating effect to the separation of compressional waves from shear waves. This is largely due to conversion between compressional and shear waves at the free surface.
Elastic seismic wave theory is well understood, particularly for a linear homogeneous isotropic earth. The surface of the earth is approximately a stress free surface. The effect of the free surface on elastic waves is well understood, as described in technical references such as Aki, K. and Richards, P., Quantitative Seismology, University Science Books (2002) or Stein, S. & Wysession, M., An Introduction to Seismology, Earthquakes, and Earth Structures, Blackwell Publishing (2003).
Prior art for separation of compressional and shear waves includes U.S. Pat. No. 2,657,373 which utilizes horizontal phase velocity as an input parameter.
Prior art to determine the direction of propagation of compressional waves includes utilizing a pressure sensor and a vector component of linear motion in the direction of the propagation. This is commonly used, as for example, in the recording and processing of Ocean Bottom Seismic (OBS) data.
Prior art to determine direction of propagation of known shear waves includes U.S. Pat. No. 4,446,541 which is applicable at a depth away from the free surface. This utilizes a combination of one linear motion vector component and one rotational vector component, both said vector components being orthogonal to the direction of propagation, and to each other.